State and Analysis of the Prior Art Known in the prior art is a twist drill for deep hole drilling, comprising a cutting portion, margins and helical flutes designed for removal of chips (SU, A, 701,743). Each of the helical flutes is arranged at an angle relative to the twist drill axis front-end wall nearest the cutting portion of the drill and conjugated with a flute bottom which is parallel with the twist drill axis and which is conjugated with a rear-end wall.
The known twist drill makes it possible to drill holes with a depth of up to 50 diameters without periodically withdrawing the drill due to an enlarged angle of flute helix (.omega.=45.degree.-60.degree.) and an inclination of the flute front-end wall relative to the twist drill axis through an angle .delta. which is greater than 90.degree., the front-end wall being made reactilinear. Productivity in drilling deep holes with such a twist drill is higher in comparison with that of the widely known twist drills used for the deep hole drilling. The combinations of correctly selected three main design parameters, i.e. rational dimensions, shape and angle of flute helix made it posible not only to eliminate periodical withdrawals of the drill but also to substantially improve the torsional stiffness of a twist drill. This is explained by the fact that the provision of a reliable removal of chips from the hole being drilled allows the dimensions of the helical flutes to be substantially reduced and the diameter of a drill core to be increased, as there is no need to have the helical flutes of a large volume for accumulation of chips. The known twist drill possesses a comparatively high stiffness combined with a reliable chip removal. This is achieved by reducing the angle of flute helix .omega. at the expense of introducing an angle of inclination .delta. of the front-end wall. The occurence of a force pressing the element of a chip against the wall of a hole made it possible to improve the conditions of the chip removal.
However, due to the fact that the front-end wall of a helical flute has throughout its full heights one and the same angle of inclination relative to the twist drill axis, the conditions of the chip removal on the entire length of the front-end wall are not equal. So, the inclination in the middle portion of the helical flute in the place of contact with large elements and curls of chips is useful, as it improves the conditions of the chip removal. The same inclination at the periphery of the helical flute in the place of contact with pulverulent particles and fine fragments of chips is unfavourable, as it impairs the working conditions of the known twist drill and reduces its endurance. This is associated with the fact that in a cross-section of the twist drill the front-end wall of such a helical flute is formed near the periphery by a convex curve, i.e. by the convolute and at the point of intersection with the margin said front-end wall forms an obtuse angle. This leads to jamming of the fine and pulverulent particles of chips in the clearance between the drill and the hole, and to their sticking to the margins, as well as impairs the twist drill endurance and increases the torque.